1. Field of the Invention
The present invention relates to a sheet feeder for feeding continuous printing paper having sprocket holes.
2. Description of the Prior Art
A sheet feeder is employed in a printer such as a serial printer which uses continuous printing paper having sprocket holes (hereafter referred to as paper). A conventional sheet feeder is disclosed in U.S. Pat. No. 4,194,660 issued Mar. 25, 1980 in the name of Seitz and U.S. Pat. No. 4,199,091 issued Apr. 22, 1980 in the name of Hubbard.
The invention of U.S. Pat. No. 4,199,091 includes a pair of frames opposed to each other, a drive gear rotatably mounted between a pair of frames, and a pin tractor belt extended between the drive gear and a hub. The pin tractor belt has a plurality of pins to be engaged with the sprocket holes of the paper at the outer circumferential surface of the pin tractor belt, and teeth to be engaged with the drive gear at the inner circumferential surface thereof.
A cover is openably provided on one of the frames for preventing the sprocket holes of the paper from deviating from the pins.
In such a conventional sheet feeder, a line of teeth provided on the inner circumferential surface of the pin tractor belt is moved with the drive gear and the hub so that the pin tractor belt is rotated.
However, according to the conventional sheet feeder, the pin tractor belt is rotated with the line of teeth in contact with flanges of the frame. At the same time, sides of the pin tractor belt contact the inside surfaces of the frames. To smoothly rotate the pin tractor belt, the pin tractor belt extends loosely between the hub and the drive gear and a gap or play is provided between the inside surfaces of the frames and the pin tractor belt.
To effect a graphic print, the sheet feeder is required to stop with a high degree of accuracy. However, the sheet feeder can only be stopped with a low degree of accuracy when provided with such a gap or play. An extreme reduction of the gap or play for improving the degree of accuracy sharply increases the friction between the pin tractor belt and the frame and between the pin tractor belt and the flanges to prevent a smooth rotation of the pin tractor belt and at worst stops the feeding of the paper due to the inability to drive the pin tractor belt.
According to U.S. Pat. No. 4,194,660, a support member is provided at the linear portion of the pin tractor belt instead of an idle wheel provided at the curvilinear portion of the pin tractor belt. The pin tractor belt disclosed in this patent is rotated while the line of teeth contacts the support. At the same time, the pin tractor belt is rotated while the side surfaces of the pin tractor contact the opposite sides of the frames. Accordingly, this feeder has the same drawbacks as the feeder disclosed in U.S. Pat. No. 4,194,660.
In the conventional sheet feeders as disclosed in both U.S. Patents, an upper circumferential surface at the belt is flush with the upper surface of the frames. The paper supported on the flush surfaces of the pin tractor belt and the frames is fed by the rotation of the pin tractor belt. However, inasmuch as the paper contacts the upper surface of the frames, a smooth rotation of the pin tractor belt is prevented.